This invention relates to an improved process for bleaching wood pulps, such as kraft pulp. More particularly, it relates to a multi-stage process in which hydrogen peroxide is used in the third stage in lesser amount than has heretofore been believed to be necessary. Appropriate temperature adjustments in the other stages, coupled with the use of less chemical in the third stage, results in a more economical process with no sacrifice in final brightness level.
Although the main objective is bleaching is to make pulp whiter and brighter to the eye, many other objectives must be kept in mind. Usually, but not always, it is desired to make the pulp white without damaging the strength characteristics of the paper to be made from the pulp. In some cases, it is desired to purify the pulp as well as to whiten it during the bleaching process by dissolving the lignin, resin, metal ions, and noncellulose carbohydrate components. For some types of dissolving pulps, it is required to bleach and simultaneously to lower the viscosity of the pulp to a predetermined level. For most purposes, it is desired to make the pulp not only white and brighter, but also to make it stable in order that it will not become yellow or lose brightness or strength on aging.
Beating pulp lowers its brightness, decreases its absorbency, and changes it in other ways, and therefore it is often required to bleach with minimum mechanical action on the pulp. Since carbonyl groups in cellulose and hemicellulose cause color reversion on aging, sensitivity to alkali and other phenomena, it is generally desired to bleach without introducing carbonyl groups into the pulp and, if possible, to diminish the content of such groups during the bleaching process.
Another very important objective, which is inseparable from all the others, is to keep the cost of bleaching to a minimum. This means bleaching with the least possible loss in pulp weight. It means selecting chemicals which permit bleaching to the required specifications with minimum cost when the amount of substance is multiplied by its unit price. Furthermore, if two bleaching processes give the same end product with the same chemical cost, the one requiring the least capital investment, the lowest maintenance cost, and minimum labor, power, and heat for operation and control would normally be chosen.
The main light-absorbing substances in wood pulp are derived from the lignin and resin components of the original wood. Therefore, to make pulp whiter, these substances must either be chemically changed in the solid state to diminish their light-absorbing characteristics, or be oxidized, reduced, or hydrolyzed to make them soluble in aqueous solutions in order to remove them from the pulp.
Bleaching with removal of lignin may be regarded in one sense as a continuation of the chemical pulping process. In order to make the fibers easily separable, the main object of chemical pulping is the removal of the lignin from the wood with minimum solution of the carbohydrate constituents, which is also the object in bleaching wood pulps. The initial removal of the bulk of the lignin by cooking is carried out with nonoxidizing substances such as alkalis, sulfides, or sulfites because they are cheap, may be used as high temperatures to speed up diffusion into the wood, and often can be recovered for reuse. However, these pulping processes cannot be carried to complete removal of the lignin without seriously degrading the carbohydrate fraction and dissolving a large part of it. Such processes leave the unbleached pulp at brightness levels varying from 20 to 65 GE.
Lignin has high reducing capacity and a suitable oxidizing agent for bleaching must be cheap, and be able to oxidize lignin readily with minimum attack on the carbohydrates. So far, no single chemical substance has been found which is cheap enough to be used commercially, and which has sufficient selectively for oxidizing lignin and dissolving it without damaging the cellulose, to bleach pulp in a single stage. Among known substances, chlorine dioxide comes closest to meeting these requirements. Although it can be used along the bleach pulp in a single stage, it is cheaper and better to use chlorine dioxide as part of a multistage bleaching system.
Chlorine is the cheapest available oxidizing agent (other than air), and it reacts very rapidly with lignin at low temperature, causing only small damage to the carbohydrate fraction of pulp. However, it does not make pulp white, since it merely chlorinates and oxidizes the lignin without dissolving much of it. The pulp is frequently darker after treatment with chlorine, and is usually a golden-orange color. Chlorine is usually used to satisfy the major part of the oxygen demand of lignin because it is cheap and saves the more expensive bleaching agent required for final whitening, and because the products of its reaction with lignin are readily soluble in dilute sodium hydroxide solution.
The second treatment in most bleaching processes is therefore an extraction with dilute aqueous sodium hydroxide solution at elevated temperatures to dissolve the chlorinated and oxidized lignin to prevent it from consuming bleaching agent in the subsequent treatment of the pulp. The caustic (NaOH) extraction stage also dissolves some resin components and some hemicellulose, the amount removed depending on the ratio of sodium hydroxide to pulp, and on the consistency, temperature, and time of treatment.
Further oxidation of the pulp for removal of the remaining lignin and whitening may be carried out with several reagents. Calcium hypochlorite has been used for more than a century, because it is cheap and quite effective. Sodium hypochlorite is preferred in some mills, chiefly because it is much simpler to prepare from chlorine and caustic soda, although some claim is made that pulp cleanliness is improved. Furthermore, the use of sodium, rather than calcium hypochlorite reduces scaling problems and facilitates recyling of the filtrate. Chlorine dioxide has replaced hypochlorite to some extent. More recently, sodium or hydrogen peroxide has found use either as a replacement for hypochlorite or as an additional step in the proces.
Multistage bleaching is now used in virtually all commercial pulp bleaching operations. In mill bleaching practice, the following abbreviations are used for various individual stages in the operation:
C=chlorination PA1 E=extraction with sodium hydroxide PA1 H=oxidation with hypochlorite PA1 D=oxidation with chlorine dioxide PA1 p=oxidation with peroxide
Further background and details regarding bleaching of wood pulp in general, and bleaching of kraft pulp, in particular, may be found in TAPPI Monograph No. 27 (1963, W. Howard Rapson, ed.) and in Rydholm, "Pulping Processes," ch. 12-17 (Interscience Publishers, 1965).
Unbleached kraft pulps have extremely low brightness levels (20-30 GE) and bleaching to a desirable brightness level of 80 GE or above has historically posed special problems. It was logical to expect that an increase in brightness of over 60 points would require large amounts of chemicals and, in order to preserve pulp strength, would also necessitate the use of a rather large number of individual stages. The most common sequences in kraft pulp bleaching processes are CEHEH, CEHDH, CEDED, and CHEDH. Of particular interest are those in which clorination and extraction with sodium hydroxide are the first two stages.
With the kraft pulp bleaching processes now known, it is possible to obtain up to 82 to 88 GE brightness levels, without sacrificing more than about 5 to 7% of the strength of the pulp. The realization of these results, however, requires careful control of the various stages. Within the first stage, the pulp is supplied with such elemental chlorine as it can absorb within the allotted time (30 to 60 minutes) at the temperature of the mill water, typically about 80.degree.-100.degree. F. (26.degree.-38.degree. C.). The amount of chlorine absorbed will vary between about 80 and 140 pounds per ton of pulp and is a function of the permanganate number (PN) of the pulp. The second stage, extraction with caustic soda, is operated at an elevated temperature, i.e., between about 135.degree. and 165.degree. F. and its function is to dissolve the chlorinated lignin and give the desired reduction in PN value. About 2 to about 3% of caustic soda is generally required for this purpose.
In the kraft pulp bleaching process, the third stage is regarded as quite critical, its traditional function being to bring the brightness level to within 10 to 20 points of its desired final value. In order to preserve the pulp strength, careful control of pH and temperature is necessary. Particularly, the temperature in the third stage, if it is oxidation by hypochlorite (H), should be no higher than about 110.degree.-115.degree. F. (43.degree.-47.degree. C.).
Hydrogen or alkaline peroxides have been used in the later stages of pulp bleaching for the purpose of brightness stability. Sequences for wood pulp bleaching such as CEHEHP, CEHDP, CEHPHP and others are known to be in commercial use. Furthermore, the use of peroxide in the third step of a bleaching process for sulfate pulp is described in an article by Christensen, "Bleaching Sulphate Pulp with Hydrogen Peroxide," Pulp and Paper Magazine of Canada 73, No. 2, 62-66 (1972), pertinent portions of which are incorporated herein by reference. The use of peroxide in the third stage of a kraft pulp bleaching process is suggested in the literature. However, because the third step of this process is expected to bring the GE brightness level up to within 10 to 20 points of the desired final brightness level--i.e., the brightness level in the third stage should be brought up to 60-70 GE--there is no economic incentive to use peroxide at this stage in the bleaching of wood pulp.